1. Field of the Invention
The present invention relates to an image forming apparatus for printers and the like.
2. Description of the Related Art
In recent years, demand has increased for printers using laser light beams, LEDs and the like as light sources and having multifunctionality insofar as being capable of duplex printing function, composite printing function, color printing function, sorter function and the like. On the other hand, demand has also increased for inexpensive, compact single-function apparatus which today can be produced having a height of only about 20 cm.
The applicant has previously proposed in U.S. Pat. No. 4,972,236 an image forming apparatus having a plurality of image forming units stacked vertically, wherein the paper supplying units and the paper discharging units of the upper and the lower image forming units are mutually connected via a paper transporting unit having a switching means for selectively changing the paper transport path.
The aforesaid image forming apparatus is capable of providing a duplex printing function, a composite printing function and a color printing function by supplying paper in various modes to a plurality of image forming units via a paper transporting unit, and is further capable of image formation via each of the respective image forming units.
U.S. Pat. No. 4,707,713 further discloses an image forming apparatus wherein a plurality of image forming units are controlled by a single controller device, such that when any of the image forming unit enters a non-functional state, said non-functional state is detected and image formation continues with the other image forming unit in accordance with identical recording conditions.
The previously mentioned image forming apparatus disclosed in U.S. Pat. No. 4,972,236 has a disadvantage inasmuch as the image forming process must unavoidably be discontinued whenever the image forming process is interrupted due to a toner empty or paper empty or like condition arises during a continuous execution of image formation of multiple sheets by an optionally selected image forming unit.
On the other hand, the image forming apparatus disclosed in U.S. Pat. No. 4,707,713 is capable of executing continuous image formation using another image forming unit when the previously described conditions occur. Nevertheless, disadvantages arise because in order to detect the discharge of the image bearing sheets to the discharge portion of the respective image forming units, it is necessary to collect the concerned sheets from the discharge portion of each image forming unit, thereby complicating the labor of the operator.